The hemophagocytic lymphohistiocytoses (HLH) comprise a growing number of inherited and non-inherited disorders of the immune system characterized by the generation of abnormal and severely damaging immune responses. HLH patients experience spontaneous and often recurring episodes of hyper-inflammation marked by the activation and expansion of CD8+ T cells and macrophages that produce copious amounts of pro-inflammatory cytokines, including interleukins (IL)-6, IL-12, and interferon (IFN)-?. Despite current treatments, up to 50% of HLH patients die due to the toxic effects of these cytokines, which further drive immune cell activation and promote a sepsis-like syndrome of hypotension, bone marrow suppression and multisystem organ dysfunction. The Janus kinases (JAKs) transduce STAT-dependent intracellular signals initiated following engagement of the Types I and II cytokine receptors, which bind a broad array of cytokines including those over-produced in HLH. Based on the critical role for pro-inflammatory cytokines in the pathogenesis of HLH, we hypothesize that pharmacologic inhibition of the JAKs will diminish inflammation and ameliorate disease. Indeed, in pilot studies in mice, we observe that the FDA-approved JAK inhibitor ruxolitinib significantly lessens the manifestations of HLH, including weight loss, splenomegaly, cytopenias, hyper-cytokinemias and hepatic inflammation. In this Exploratory Grant Investigation (PA-13-315), we plan to further test this hypothesis by determining the therapeutic effects of ruxolitinib and deciphering its underlying mechanism(s) of action. Using 2 complementary murine HLH models, we will refine the dosing and schedule of ruxolitinib administration and assess its efficacy when used as a single agent or in combination with more conventional HLH therapies (i.e. dexamethasone, etoposide). We will complete immunologic studies to examine how ruxolitinib influences patterns of immune cell activation and cytokine production, JAK and/or STAT phosphorylation and STAT-dependent gene transcription. Despite the inadequacy of current HLH therapies, there have been no significant advances in treatment in more than a decade. Through the proposed studies, we intend to improve the outcome for HLH patients by targeting JAK-dependent cytokine networks, a new and rational approach to treatment. To facilitate the successful completion of these studies, we have gathered a strong investigative team that includes Drs. Kim Nichols, Ed Behrens and David Teachey (The Children's Hospital of Philadelphia), John Wherry (The University of Pennsylvania) and Michael Jordan (Cincinnati Children's Hospital Medical Center). Together, the members of this team are experts in the biology of HLH and cytokine signaling, pre-clinical modeling of human diseases and the bench-to-bedside translation of research findings. Therefore, should this study continue to demonstrate a beneficial effect, we anticipate that its results can be readily transitioned to te clinic in the form of a future ruxolitinib-based trial for children and adults with HLH.